Delusion

A delusion, in everyday language, is a fixed belief that is either false, fanciful, or derived from deception. Psychiatry defines the term more specifically as a belief that is pathological (the result of an illness or illness process). As a pathology, it is distinct from a belief based on false or incomplete information, apperception, illusion, or other effects of perception.

Delusions typically occur in the context of neurological or mental disorder, although they are not tied to any particular disease and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders and particularly in schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.